Draining and Refilling a Central Heating System Properly
Draining a heating system is straightforward in principle and easy to get wrong in ways that cause months of trouble afterwards.
Draining a heating system is straightforward in principle and easy to get wrong in ways that cause months of trouble afterwards. The common mistakes are draining without a plan for the sludge that comes with the water, refilling too fast so the system airlocks, and forgetting that inhibitor leaves with the water and has to be replaced. Done properly, the system comes back quieter and better protected than before.
When draining is actually necessary
Replacing a radiator, moving pipework, fitting a magnetic filter, replacing a pump or valve where isolation is not possible, or cleaning a heavily sludged system. It is not necessary for bleeding air, for balancing, or for most boiler component work where the boiler can be isolated. Draining only the section you need, using radiator valves or isolation valves, saves time and keeps most of the treated water in the system.
Before you open anything
Turn the heating off and let the system cool — draining hot water is a scalding risk and stresses joints. Isolate the electrical supply to the boiler. Close the filling loop and, on a vented system, tie up the ball valve in the feed and expansion tank or close its supply. Identify the drain-off point, which is usually at the lowest point of the system on a downstairs radiator, and have a hose that reaches somewhere sensible to discharge.
Draining without making a mess
Attach and clip the hose securely to the drain-off before opening it, because system water is black, stains, and will find carpet unerringly. Open the drain-off, then open bleed valves on upstairs radiators first and work down to let air in behind the water — the system will barely drain without it. Expect the process to be slow at the end, and expect debris to be present, which is exactly why the hose termination should not be a flowerbed you care about.
Refilling and getting the air out
Close the drain-off and all bleed valves, then refill slowly. Filling fast pushes air into pockets that then take ages to clear. On a sealed system, refill via the filling loop to the correct pressure band, usually with the system cold, and disconnect the loop afterwards. On a vented system, let the tank refill and the ball valve shut off naturally. Then bleed each radiator from the lowest upwards, topping the pressure back up as it drops.
Inhibitor is not optional
Every litre of water that left the system took inhibitor with it, and a system refilled with plain water is unprotected against corrosion from that moment. Correct dosing for the system volume goes in as part of the refill, not as an afterthought weeks later. If the system was drained because it was dirty, a cleaner may go in first, be circulated, then be drained and replaced with fresh water and inhibitor.
Checks after refilling
Run the system up to temperature and check every joint you disturbed, plus a few you did not, because expansion finds weak points. Re-check pressure hot and cold. Bleed again after a day or two as dissolved air comes out of solution. Check the magnetic filter after a few weeks — a system that has been disturbed usually releases debris, and clearing it early protects the pump and heat exchanger. It is also worth walking round every radiator once the system is fully hot to confirm nothing has been left airlocked or accidentally closed down, since a lockshield knocked while working behind a radiator is easy to miss and shows up later as one cold room. If the system was balanced before the drain-down, expect to re-check that balance afterwards rather than assuming it survived.
Questions we get asked
Do I need to drain the whole system to change one radiator?
Usually not. Closing both radiator valves and draining just that radiator keeps the rest of the system full and treated.
Why won't my system drain properly?
Almost always because air cannot get in behind the water. Opening upstairs bleed valves lets it drain freely.
How much inhibitor does a system need?
It depends on system volume, so dosing follows the product's stated coverage rather than a fixed bottle per house. Under-dosing leaves the system partly unprotected.
Why do radiators need bleeding again a few days later?
Dissolved air comes out of solution as the system heats and cools, collecting at high points. A second bleed after a couple of days is normal.
Is black water from the system a bad sign?
It indicates magnetite from internal corrosion. Some is expected in an older system, but heavy contamination suggests cleaning and a magnetic filter are overdue.